Streptococcus pneumoniae (Pn) is an extracellular bacterium that is a major cause of global morbidity and mortality. Systemic adaptive immunity to Pn is mediated by antibody, especially IgG specific for the capsular polysaccharide (PS), but also for bacterial proteins. Our long-term goal is to elucidate the cellular mechanisms that underlie the distinct differences that exist between in vivo anti-PS and anti-protein Ig responses to intact Pn, as a prerequisite to the development of improved vaccines. In contrast to the current dogma, based on using purified PS antigens, that IgG anti-PS responses are T cell-independent, we demonstrated that the IgG anti-PS response to intact Pn is heavily dependent on CD4+ T cells but nevertheless, still exhibits striking differences in kinetics, generation of memory, and the functional roles of dendritic and T cells relative to the co-induced anti-protein response. In light of these data, the central hypothesis that underlies our proposed research is that differential B cell receptor signaling and/or involvement of functionally distinct B cell subsets are the key parameters that distinguish physiologic anti-PS and anti-protein Ig responses. We will demonstrate that these parameters differentially impact on 1) the temporal compartmentalization of responding B cells within the spleen, and 2) the cellular interactions of the responding B cells with other immune cell types. As a result we will provide a mechanistic basis that will elucidate the observed differences in anti-PS and anti-protein responses. The specific aims are to: 1. Determine the nature and relationships of B cell and dendritic cell subsets, and CD4+ T cells that differentially mediate in vivo anti-PS and anti-protein Ig isotype responses to systemic immunization with intact Pn. We will utilize high speed electronic cell sorting and adoptive transfer of wild-type and genetically altered immune cells combined with ELISPOT and ELISA analyses of antigen-specific Ig isotype production to accomplish this aim. 2. Determine the mechanism by which B cells, DCs, and CD4+ T cells differentially orchestrate anti-PS and anti-protein responses within a spatiotemporal context. We will utilize B cells from BCR knock-in mice with Ig specificity for Pn-derived PS or protein antigens and T cells from CD4+ TCR transgenic mice with specificity for a Pn-derived protein to accomplish this aim. These cells will be used to conduct in vitro functional studies and confocal microscopic analyses of splenic tissue sections post-immunization. These studies are the first to systematically determine the mechanisms that distinguish anti-PS from anti-protein responses to an intact bacterium and provide novel basic immunologic insights with direct relevance to the development of anti-bacterial vaccines. [unreadable] [unreadable] [unreadable]